Fabrication of BaO-Al₂O₃-SiO₂ environmental barrier coatings and characterization of their corrosion resistance

BaO-Al₂O₃-SiO₂ (BAS) sol was prepared by the citric acid sol-gel method. The BAS sol was coated onto carbon fiber (C_f)/SiC-SiC substrate directly using a brush-coating process. The corrosion resistances of samples with and without BAS coating were determined under 1250°C, 50% (in volume) H₂O-50% O₂ and an atmospheric pressure environment. The flexural strengths of samples before and after the corrosion were tested. The characteristics of BAS sol-gel as well as BAS coating were measured by Fourier transform infrared spectroscopy, thermogravimetry-differential scanning calorimetry, X-ray diffraction, and scanning electron microscopy. The results indicate that the BAS sol synthesized at a pH value range of 4-5 is uniform, transparent and stable with the mole ratios of citric acid to metal ion Ba²⁺ and Al³⁺ of 2 and 3, respectively. The thickness of BAS coating for one deposition is approximately 10μm. After the application of four layers of coating, the thickness of the BAS coating is about 50μm. The C_f/SiC-SiC composites with BAS coating can remain 88% of their original flexural strength even after 200 h corrosion. Compared with C_f/SiC-SiC composites without BAS coatings, those with BAS coatings do not have any obvious mass loss after corrosion.